That's not necessarily true. If you're a certified non-profit, you can get free Slack with unlimited message history for up to 250 users (https://slack.com/help/articles/204368833).
The thesis defended in the article is that Slack is not suited to communicate with the userbase of open source projects.
Capping a basic feature such as collecting message history at 250 users is a limitation that supports the author's thesis, as clearly it demonstrate that Slack is unable to handle the communication needs of a moderately popular open source project.
And I don’t even think it’s applicable - in the community I’m in we decided we didn’t want an eternal log of everyone’s discussions. More formal discussions in PRs and emails are recorded, but we rather let conversations fade away after some time.
EDIT:
Answered my own question: up to 85% discount, which is very nice, but still might be infeasible for many projects/communities.
Isn't Slack's cheapest plan around 8$ per user per month?